Touch-screen technology combines display and input functionality in a unified, intuitive user interface (UI). Available in many sizes and form factors, and leveraging various sensor technologies, the touch screen has become a ubiquitous UI modality. Nevertheless, touch-screen technology has inherent limitations. For instance, while a touch-screen display may be viewable from a great distance, it can receive input only when the user is nearby. Furthermore, touch screens subject to normal use quickly become smudged by oils and other contaminants from the user's hands. Smudging degrades display quality and necessitates frequent cleaning.